Overview
Reinforcement chairs, or bar supports, ensure proper positioning of reinforcement and preservation of the concrete cover. Their selection influences structural performance, long-term durability, corrosion risk, fire safety, and construction quality, particularly in demanding exposure conditions. This article outlines how to select reinforcement chairs based on environmental class, material compatibility, tie wire corrosion, load and fire behavior, and national and international codes.
Bar Chair Selection: Key Factors
Appropriate chair selection must consider site-specific conditions and service environment. Critical selection factors include:
- Exposure class and design life expectations (AS3600 Cl. 4.10, Cl. 5.6.4)
- Mechanical and thermal performance of chair material
- Placement technique, movement resistance, and loading during construction
- Durability of embedded wire components (if used)
- Compatibility with reinforcement and surrounding concrete
Chair Material Options: Comparative Overview
| Material | Advantages | Limitations | Best Use Cases |
|---|---|---|---|
| Plastic | Non-corrosive, economical, dimensionally stable for standard loads | Melts or softens in fire, lower load capacity than concrete or steel | Slabs and beams in dry interiors, general-purpose low-risk zones |
| Galvanized Steel | High compressive and tensile strength, moderate corrosion resistance | Must not breach cover zone unless properly detailed and passivated; may trigger dissimilar metal corrosion if near stainless reinforcement | Heavily loaded structures with deep covers or isolated embedment, prefabricated mats with central chair supports |
| Stainless Steel | Superior corrosion and heat resistance, long life expectancy | High material cost, limited availability for large projects | Marine, coastal, chloride-prone, or fire-rated infrastructure such as tunnels and treatment plants |
| FRP | Electrically non-conductive, corrosion-free, non-magnetic | Brittle under impact, loses integrity in fire unless specially treated | Hospitals, airports, MRI facilities, water treatment structures |
| Concrete (with/without tie wire) | Fireproof, dense, matches thermal and material properties of the structure | Heavy, prone to corrosion if steel wire is near surface; must meet AS/NZS 2425 dimensional tolerances | Fire-rated zones, bridges, marine structures; ideal with plastic clips or wire-free |
Tie Wires and Concrete Chairs: Durability Implications
Where reinforcement spacers use tie wires, their durability under moist, chloride-laden, or aggressive environments must be addressed. AS3600 assumes that non-structural embedded metal can become corrosion initiators if insufficiently covered. Key implications include:
- Wires placed near the face of a concrete block may breach cover during formwork contact
- Carbon steel wire (black) rapidly corrodes under alkaline pore water exposure
- Galvanized wire slows corrosion onset but must be isolated from chloride ingress
- Stainless steel wire is preferable in exposure classes A1–A2 and U (AS3600 Table 4.10.3)
- Plastic clips or bar weight should be preferred where wire-free detailing is feasible
Corrosion Science and Galvanic Compatibility
Galvanic corrosion between dissimilar metals requires three conditions: contact, electrolyte, and potential difference. Good cover design mitigates these risks:
- Thick, dense, well-compacted concrete prevents chloride and oxygen penetration
- Proper mix design maintains passive layer on steel
- Well-separated wires or chair metals reduce potential for galvanic coupling
Use of galvanized steel in contact with black reinforcement or stainless steel is acceptable when design cover is met and placement avoids direct contact in wet zones. Refer to Galvanizers Association of Australia guidance and AS/NZS 4680 for coatings.
Fire Resistance and Thermal Stability
Chair materials react differently to fire exposure. Fire-critical structures (e.g., tunnels, chemical storage) should avoid polymers:
- Plastic chairs soften or burn at relatively low temperatures (~200–300°C)
- Concrete and stainless steel chairs retain full support even above 900°C
- FRP loses strength in fire unless specified as fire-treated composite
Risk, Compliance, and Load-bearing Guidance
Compliance with AS3600 and international codes involves structural, durability, and placement checks:
- Chair load ratings should meet the dead + live load from rebar mats during fixing (AS/NZS 2425 outlines minimum performance classes)
- Exposure class and cover must be selected per AS3600 Table 4.10.3 and 5.6.4
- Embedded accessories (spacers, tie wire) must not interfere with passive layer or cause localized rusting
Cover Requirements and AS3600 Guidance
AS3600 sets minimum cover depths based on durability, fire resistance, and exposure. Typical minimums include:
- 25 mm – interior dry zones (C1–C2)
- 40 mm – exterior moderate zones (B1–B2)
- 50–65 mm – marine splash and tidal zones (U, A1–A2)
Allowances must include bar diameter, construction tolerance, and site compaction quality. Tools like the Promaxa Cover Calculator support precise design based on AS3600 parameters.
Reinforcement Tolerance Verification
To ensure bar positioning complies with AS3600 tolerances for cover and bar spacing, the Promaxa Reinforcement Positioning Tolerance Calculator assists QA teams in assessing field placement limits and clearance margins.
Conclusion
Chair selection must be treated as an integral design decision—not merely a construction detail. Fire, corrosion, mechanical load, and compliance risk all originate from poor chair or wire specification. In aggressive exposures, stainless steel or plain concrete supports should be prioritized. For general use, plastic chairs may suffice if not subjected to thermal or chemical stress. Galvanized chairs are structurally robust but must be detailed carefully when embedded. Ultimately, the chair system must complement the exposure class, construction method, and reinforcement layout while ensuring durable and code-compliant outcomes.